In semiconductor devices (such as a VLSI) there is at least one junction and usually more than one junction, i.e. the interface between different semiconductor materials. The junction is the weakest section of the semiconductor device from the standpoint of being vulnerable to damage by heat. In the prior art, the computer industry has provided large fans, or at least fans which can effect large air mass flow over semiconductor circuits to keep them cool. The computer industry has determined empirically the temperature at which particular semiconductor junctions are damaged and what the air mass flow must be to prevent such damage. Accordingly the air mass movers, (i.e. the fans), move the air at the maximum air flow necessary to prevent damage, even though the heat being generated by the semiconductor generates a temperature which is far lower than the temperature for which maximum air flow is required. Such a prior art arrangement is wasteful, in so far as power (energy) is concerned, since the system could be run at a lower air mass flow rate if indeed the temperature generated by the semiconductor did not warrant the maximum air flow. In addition in the prior art, the fans effecting the air mass flow produce high acoustical noise and this infirmity is particularly troublesome since computers are being moved from the "computer room" to offices where personnel are trying to do all kinds of work and where acoustical noise is undesirable. The present device enables a fan to be driven only to the extent necessary which results in a savings in energy and which results in a reduction in acoustical noise.